docs: def calendar_dates(self, val):
          """
          Update ``self._calendar_dates_g``
          if ``self.calendar_dates`` changes.
          """
          self._calendar_dates = val
          if val is not None and not val.empty:
              self._calendar_dates_g = self._calendar_dates.groupby(
                  ["service_id", "date"]
              )
          else:
              self._calendar_dates_g = None